Meaning of “coo” - Learner’s Dictionary

coo

verb [ I ] uk us /kuː/ (present participle cooing, past tense and past participle cooed)
MAKE SOUND

to make a soft, low sound, like a pigeon (= large, grey bird)

SPEAK

to speak in a soft, low voice

(Definition of “coo” from the Cambridge Learner’s Dictionary © Cambridge University Press)